Conventional motor drive apparatus used to drive electric vehicles and hybrid vehicles are shown in JP Patent 3683405 and JP Patent Publication 2006-112489A. The motor drive apparatus disclosed in JP Patent 3683405 includes a motor, and a transmission comprising a belt type continuously variable transmission (CVT) or a planetary gear transmission in which the rotational speed of the motor is changed and transmitted to a differential gear assembly, through which the power is distributed to right and left auxiliary (rear) drive wheels.
The motor drive apparatus disclosed in JP Patent Publication 2006-112489A includes a motor, and a transmission comprising a planetary gear transmission in which the rotational speed of the motor is changed and transmitted to a differential gear assembly, through which the power is distributed to right and left auxiliary (rear) drive wheels.
With the motor drive apparatus disclosed in JP Patent 3683405, since the torque transmission path from the motor to the auxiliary drive wheels is always closed, while the vehicle is being driven solely by the engine, the transmission and the motor are rotated by the rotation from the auxiliary drive wheels. This leads to a waste of energy. Especially if the motor is a permanent magnet type synchronous motor, large power is lost to rotate such a motor compared to an induction motor.
With the motor drive apparatus disclosed in JP Patent Publication 2006-112489A, the transmission can be switched over between low-gear, high-gear and neutral positions by sliding a key provided in the transmission. Thus by switching the transmission to the neutral position, it is possible to prevent the transmission and the motor from being rotated by the auxiliary drive wheels. But in order to couple the ring gear of the planetary gear assembly to the casing or to couple the ring gear to the sun gear for gear change, it is necessary to synchronize the two members to be coupled together, i.e. it is necessary to sufficiently reduce the difference in rotational speed between the two members. A long time is necessary for such synchronization of the two members. During such synchronization, the vehicle travels by inertia. This deteriorates drivability of the vehicle and also lowers its commercial value.